Just stumbled across this thread and thought I'd share my findings so far.
I have a strange scenario where my older SD cards - a 2017 Sandisk Extreme Pro 64GB 95mb/s and a 2016 Sandisk Extreme 64GB 90mb/s - work perfectly in my internal iMac 5k SD reader (late 2015 running Catalina), but none of the 4 brand new cards I bought this week work in the internal reader, but DO work in my external USB 3 reader.
I purchased 2x 64GB Extreme Pro 170mb/s cards (the updated model of the 95mb/s cards) and they work fine in my external reader, but when plugged into the internal reader, they always take 2-3 goes to show up, when they do, the finder crashes and images can't be dragged to desktop, it just gives an error -36 message. Then you have to force eject every time even though it's no longer in use. So I returned the 170mb/s cards on Amazon, got a next day delivery of 2x Lexar 128GB 633x - exact same problem - works in external reader but not on internal.
But it's not just these brand new cards - I have another 64GB Extreme Pro 95mb/s I bought back in 2018 that's never worked consistently but forgot to return in time - it's experiencing the exact same symptoms, and it looks IDENTICAL to my working 95mb/s card from 2017.
All cards are exFAT, always formatted in camera, and it's only my old cards that can be read by the internal reader.
If anyone wants to see the problem in action, I recorded a sped-up 3 minute video demonstrating the problem. https://www.youtube.com/watch?v=rnXnUZFXY8M
Sandisk got back to me saying 'if the cards work in an external reader it's obviously not the card that's the problem', and Apple support got me to reset the various PRAM etc then run a fresh install of Catalina, all to no avail, then said next thing to try would be a complete erase and reinstall, and if that fails we know it's a 'hardware issue'. The million$ question for me is why do my older cards still work?
Also interestingly - the new cards also don't work in my 2012 15" Retina MacBook Pro with Mojave, last year's OS, so the problem could be more than Catalina in my case.
I'll be following this thread carefully. For now I'll have to stick to using external card readers which I'd rather not have to do when travelling with my laptop because I'm bound to forget it one of these times, I'd also rather keep the extra USB port for other things, but at least I have a workaround just now.
